1. Field of the Invention
The present invention relates to a ferroelectric liquid crystal ("FLC") device and a liquid crystal apparatus containing the same. More specifically, the present invention relates to a FLC device structure which prevents a reduction in contrast due to fluctuations of liquid crystal molecules.
2. Description of the Related Art
Clark and Lagerwall have proposed a display device in which the property of anisotropic refractive index of the ferroelectric liquid crystal is advantageously used with a polarizing element to control light transmission (U.S. Pat. No. 4,367,924). In general, ferroelectric liquid crystals have a non-helical structure in a chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature range. Such a ferroelectric liquid crystal can be in either a first stable optical state or second stable optical state depending on an applied electric field, which will be maintained even if the applied electric field is removed. This property is called "bistability". Furthermore, ferroelectric liquid crystals can respond quickly to a change in the electric field. Therefore, ferroelectric liquid crystals are expected to be used in a variety of high-speed and/or storage-type display devices. Their properties especially allow them to be used in high-resolution and large-size display devices. Japanese 10 Laid-Open Patent No. 03-252624, discloses a technique to achieve high contrast and high-speed operation in a large-size high-resolution display using a chiral smectic ferroelectric liquid crystal.
First, methods of achieving high contrast will be discussed. In general, the transmittance (I/I.sub.0) of a liquid crystal using the property of birefringence under the crossed-Nicol condition is given by EQU I/I.sub.0 =sin 4.theta..sub.a sin.sup.2 (.DELTA.n d/.lambda.).pi.
where I.sub.0 is the intensity of incident light, I.sub.0 is the intensity of transmitted light, .theta..sub.a is an apparent tilt angle, .DELTA.n is anisotropy in refractive index, d is the thickness of a liquid crystal layer, and .lambda. is the wavelength of incident light. It can be seen from the above equation that it is preferable to employ 22.5.degree. as the apparent tilt angle .theta..sub.a (described later in detail) to obtain a high transmittance, thereby achieving improved display quality.
In the case of a liquid crystal oriented in a non-helical structure as described above and as utilized herein, the "apparent tilt angle" .theta..sub.a is given by an angle between the average molecular axis of liquid crystal molecules in the first orientation state and the average molecular axis of liquid crystal molecules in the second orientation state.
However, in a conventional ferroelectric liquid crystal device having a non-helical orientation structure formed by orienting the liquid crystal using a rubbing-treated tilt angle .theta..sub.a (one-half the angle between molecular axes of polyimide film as an orientation control layer, the apparent molecules in two stable states) becomes smaller than the cone angle of the ferroelectric liquid crystal (represented by .THETA. in FIG. 2A and B, which is half the vertical angle of a triangular pyramid shown). Typically, the apparent tilt angle is thus in the range from 3.degree. to 8.degree., and thus the transmittance I/I.sub.0 is as low as 3% to 5%.
In general, smectic liquid crystals have a layer structure, and the distance between layers shrinks during the transition from the SmA phase to the SmC phase or SmC* phase. Thus, liquid crystal layers denoted by 11 in FIG. 1 are bent at the center between upper and lower substrates. Liquid crystal layers can bend in two different directions. In an orientation state (C1 orientation state) which occurs just after the transition from a high-temperature phase to an SmC* phase, liquid crystals are oriented in the manner shown in region 12 in FIG. 1. If temperature is lowered further, liquid crystals assume another orientation state (C2 orientation state) which appears in mixture with the C1 orientation state, in which liquid crystals are oriented in the manner shown in region 13 in FIG. 1. It is known that C1-oriented liquid crystals can be in two stable low 10 contrast states (hereafter referred to as "splay" states) in which directors of liquid crystals are twisted between upper and lower substrates. In addition to the splay states, the present inventors have also determined that there are two high contrast stable states (hereafter referred to as "uniform" states).
If an electric field is applied, transition occurs between these states. When small amplitude positive and negative electric field pulses are applied, transition occurs between two splay states. On the other hand, if large amplitude positive and negative electric field pulses are applied, transition occurs between two uniform states.
In the uniform states, the apparent tilt angle .theta..sub.a becomes large. Accordingly, it is possible to realize liquid crystal display devices which exhibit higher brightness and higher contrast than conventional devices. Therefore, use of a liquid crystal is in the C1 orientation state over the entire display area of a display device, and use of two high-contrast states in the C1 orientation for displaying white and black, provides a display device with higher performance than conventional display devices.
In regions near substrates, directors in C1 and C2 orientations are on cones 21 in FIGS. 2(A) and 2(B), respectively. As is well known, if rubbing is carried out, liquid crystal molecules lying at substrate interfaces achieve a pretilt angle in such directions that head portions of liquid crystal molecules rise relative to the rubbing direction (shown by A in FIGS. 2(A) and 2(B)). That is, end portions of liquid crystal molecules float relative to the rubbing direction.
From the above, the following relationship holds among the cone angle .THETA., pretilt angle .alpha., and layer tilt angle .delta.(the angle between a liquid layer and the normal of a substrate);
.THETA.+.delta.&lt;.alpha. for C1 orientation; PA1 .THETA.-.delta.&gt;.alpha. for C2 orientation. PA1 a pair of substrates comprising electrodes for applying a voltage, said pair of substrates having been subjected to a uniaxial orientation treatment, and being oriented at a distance opposite to each other such that the uniaxial orientation axes cross each other at a predetermined angle; said display device further comprising a liquid crystal material disposed between said exhibiting at least two stable states and pair of substrates, said liquid crystal material having a cone angle, a pretilt angle, a tilt angle and an apparent tilt angle, wherein: PA1 said liquid crystal material having an orientation state which satisfies the conditions represented by EQU .THETA.&lt;.alpha.+.delta.,.delta.&lt;.alpha., and .THETA.&gt;.THETA..sub..alpha. &gt;.THETA./2 PA1 in which .THETA. denotes said cone angle, .alpha. denotes said pretilt angle, .THETA. denotes said tilt angle and .alpha..sub..alpha. denotes said apparent tilt angle. PA1 wherein the phase of said liquid crystal changes from isotropic phase via cholesteric phase and smectic A phase to chiral smectic C phase as said liquid crystal cools from a high temperature and said liquid crystal has a temperature range in which said cholesteric phase and said smectic A phase exist at the same time.
Therefore, if the following condition holds, it is possible to obtain the C1 orientation without C2 orientation; EQU .THETA.-.delta.&lt;.alpha. or .THETA.&lt;.alpha.+.delta. (I)
Furthermore, by simple considering the torque exerted by an applied electric field on liquid crystal molecules lying at an interface between a substrate and a liquid crystal layer, it is understood that liquid crystal molecules can be moved from one location to another if the following relationship holds. EQU .alpha.&gt;.delta. (II)
Thus, it can also be understood that if equations (I) and (II) are both satisfied, it becomes possible to achieve more stable formation of the C1 orientation state without transition from C1 to C2. Experiments have revealed that when both equations (I) and (II) are satisfied, the apparent tilt angle .theta..sub.a increases from 3.degree.-8.degree. to about 8.degree.-16.degree., and furthermore the following relationship holds between the cone angle .THETA. and tilt angle .theta..sub.a of a liquid crystal: EQU .THETA.&gt;.theta..sub.a &gt;.THETA./2 (III)
As described above, it has become clear that if equations (I), (II), and (III) are satisfied in a ferroelectric liquid crystal device, then it is possible to realize a high-quality display device which provides high-contrast images.
Next, methods of achieving high-speed operation in a liquid crystal display will be discussed.
If a chiral smectic liquid crystal device is used, it is possible to realize a display having much higher resolution and having a much larger display area than conventional displays such as cathode ray tubes ("CRTs"), twisted nematic ("TN") type liquid crystal displays, etc. However, as the display area becomes larger and the resolution becomes higher, the frame frequency (the period required to form one picture) becomes lower. This means that there is a problem that operation speeds become too low for image rewriting, character editing, smooth scrolling of graphics images, cursor movement, etc.
One technique to solve this problem is disclosed in Japanese Laid-Open Patent Applications Nos. 60-31120 and 1-140198. In this technique, a display is constructed with: a display panel having scanning electrodes and information electrodes arranged in a matrix; means for selecting all or a predetermined number of scanning electrodes (referred to as "entire-area" writing); and means for selecting fewer than all or the predetermined number of scanning electrodes (referred to as "partial" writing); whereby moving portions of an image are displayed at a high speed by partial writing while maintaining capability of entire writing. Taking the above into account, if a liquid crystal device satisfying the conditions (I), (II), and (III) is driven in partial-writing, it is possible to achieve high-speed display a high-contrast image in a high-resolution large-size display device. However, if such a liquid crystal device is driven by an applied voltage in the above-described manner, fluctuation occurs in liquid crystal molecules due to pulses applied to the liquid crystal device during non-selected time periods. These pulse fluctuations result in a great reduction in contrast and thus great degradation in display quality. In fact, in extreme cases, movement of the liquid crystal material occurs and the device is destroyed.
In a conventional display panel using a chiral smectic liquid crystal, scanning and information electrodes are arranged in a matrix form between upper and lower substrates so that a predetermined voltage is applied to the liquid crystal. FIG. 3 illustrates an example of an applied voltage waveform, in which the waveform 31 is used to display white, and the waveform 32 is used to display black.
In waveforms 31 and 32, electric fields required to display white and black at each matrix point (the intersection points between the scanning and information electrodes) are found in periods 34 and 35, respectively. During the other periods (non-selected periods), positive and negative electric fields having such small magnitudes that no rewriting occurs are alternately applied. 0f course, the total time of non-selected periods is much longer than the selected periods during which an electric field is applied to an element at matrix point.
Since positive and negative electric fields too small to invert liquid crystal molecules are applied alternately even during the non-selected periods, at the moment an electric field is applied to liquid crystal molecules having a polarity opposite to the spontaneous polarization of such molecules in one stable, they are subjected to force which tries to invert the liquid crystal molecules into the other stable state. In some cases, as a result, these liquid crystal molecules move by a small amount on cone 21 shown in FIG. 2(a) or 2(b) in the inversion direction. At the moment immediately thereafter, the liquid crystal molecules return to the first stable state by an applied field having an opposite polarity. In this way, liquid crystal molecules are inverted repeatedly, causing "fluctuations" of liquid crystal molecules. These fluctuations can produce a defect through which light can pass when the point should display black. As a result, contrast is reduced and degradation in display quality occurs.